1. Field of the Invention
This invention concerns a gas turbine engine combustion systems and also concerns gas turbine engines provided with such systems.
2. Detailed Description of the Related Art
It is known to improve atomization and placement or positioning of liquid fuels within gas turbine engine combustion chambers by the use of electrodes located so as to impart electrostatic charge to the fuel droplets. For example, U.S. Pat. No. 4,439,980 discloses a gas turbine engine wherein fuel is injected through a spray injection nozzle towards an electrode in a combustion chamber so that after it has left the injection nozzle, the fuel becomes electrostatically charged, and the strength of the electric field is adjusted to provide a spray characteristic said to produce an optimum engine performance.
The inventor believes that further increased control of fuel placement, vaporization and combustion intensity is desirable. This would lead to greater combustion stability, particularly at low fuel injection rates, and lower emission of pollutants from engines. In particular, it is desirable to still further improve the already good low emissions and stability characteristics of gas turbine engine combustors of the lean burn type employing combustion xe2x80x9cpre-chambersxe2x80x9d, which are of smaller volume and cross-sectional area than a main combustion chamber into which they discharge. These pre-chambers receive preswirled, premixed liquid fuel/air mixtures for combustion therein from xe2x80x9cpreswirlersxe2x80x9d, the latter comprising for example circular arrays of vanes defining passages therebetween which are configured to impart to the fuel/air mixture a swirling motion about a longitudinal axis of the pre-chamber. In one known efficient class of preswirler with which the inventor is particularly concerned, the fuel/air mixture enters a cylindrical pre-chamber from preswirler passages at the pre-chamber""s upstream end, the preswirler passages being oriented such that the fuel/air mixture enters the pre-chamber with a mainly tangential component of velocity, though a radial velocity component is also present for a desired amount of penetration of the mixture towards the pre-chamber combustion region.
An object of the invention is therefore to provide a gas turbine engine combustion system in which one or more of fuel placement, vaporization and combustion intensity may be more accurately controlled to produce an improved combustion performance.
According to the invention, a gas turbine engine combustion system comprises
a combustion main chamber,
a combustion pre-chamber upstream thereof and opening into the main chamber, the pre-chamber being of smaller flow area than the main chamber and being disposed about a longitudinal axis,
a burner face at an upstream end of the pre-chamber,
a preswirler assembly comprising a plurality of preswirl passages communicating with the upstream end of the pre-chamber for supplying a preswirled air/fuel mixture to the pre-chamber, the preswirl passages being disposed about the longitudinal axis,
atomizing injection nozzles located in the preswirl passages to inject atomized fuel thereinto, each said injection nozzle including a first electrode means operable to selectively electrostatically charge the first electrode means at a pre-determined polarity thereby to impart electrostatic charge to the atomized fuel,
second electrode means forming at least portions of the preswirl passages, and
means operable to selectively electrostatically charge the second electrode means at the same polarity as the first electrode means, thereby to repel the atomized injected fuel from the preswirl passage portions.
The pre-chamber is preferably of cylindrical form, with the preswirl passages extending substantially tangentially to the periphery of the pre-chamber.
Each preswirl passage may have at least one atomizing injection nozzle located therein and each first electrode means preferably comprises a sharp charge-emitting edge disposed around an exit of its corresponding atomizing injection nozzle.
Preferably the second electrode means comprises walls of the preswirl passages and, in fact, it is convenient if the preswirler assembly itself comprises the second electrode means.
Third electrode means may be provided in association with the burner face, and means may be provided for holding the third electrode means at a potential with respect to the electrostatically charged fuel such that the fuel is biased towards the third electrode means. At least a portion of the burner face, preferably a substantially central portion, may comprise the third electrode means.
A preferred embodiment provides fourth electrode means extending peripherally of the third electrode means, and means to selectively electrostatically charge the fourth electrode means at the same polarity as the charged fuel. The first and fourth electrode means may be connected in an electrically conducting manner whereby said first and fourth electrode means are at the same potential. Furthermore, a fifth electrode means may be interposed between the third and fourth electrode means, means being operable to selectively electrostatically charge the fifth electrode means at a polarity opposite that of the charge on the fuel. Advantageously, a fuel ignition means is disposed in the fourth or fifth electrode means.
The combustion system may be further provided with sixth electrode means comprising at least a portion of the pre-chamber, and means to selectively electrostatically charge the sixth electrode means at the same polarity as the charge on the fuel. Preferably, a wall region of the pre-chamber comprises the sixth electrode means.
In a gas turbine engine comprising an aforesaid combustion system according to the invention, repulsion of the fuel by the second electrode means tends to keep the fuel off walls of the swirler assembly. Where the sixth electrode means is provided in the pre-chamber, repulsion of fuel thereby tends to focus fuel flow closer to the axis of the pre-chamber and away from the wall of the pre-chamber. Such control of fuel flow admits improvements in engine operation particularly at ignition or at low load, for example load shedding operation, and because the fuel is in atomized liquid or droplet form, keeping it off the swirler assembly or the pre-chamber wall tends to avoid coking the assembly or the pre-chamber. If ignition means is provided in the burner face, fuel attracted thereacross towards the burner face has an improved chance of ignition and this can also improve operation of the gas turbine engine.
Further aspects of the invention will be apparent from the following description and claims.